1. Field of the Invention
The present invention relates to organic light-emitting devices. More particularly, the invention provides a kind of encapsulation method of the organic light-emitting devices and the application of this method.
2. Description of the Related Arts
Recently, with the development of multimedia technology and the coming of information society, the desire for high performance flat panel displays becomes more and more emphasized. Three recently developed kinds of display technology, i.e. plasma display, field emission display and organic light-emitting display, can make up for shortcomings of the CRT and LCD to a certain extent. Selected from these display technologies, organic light-emitting devices (OLEDs) show many advantages such as self-emission, low voltage operation, all-solid state construction, wide view angle and full color. The OLEDs also show a quick response speed of up to 1000 times that of an LCD display and its manufacturing cost is relatively low compared to an LCD display with same distinguishability. Thus, OLEDs show a great foreground in the display field.
In 1987, C. W. Tang et. al. of Kodak Company reported a light-emitting diode with a double-layer structure of organic thin films, which is prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium-tin-oxide anode and an alloyed Mg:Ag cathode. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W) and brightness (>1000 cd/m2) were achievable at a drive voltage of 10 V. (C. W. Tang, Applied Physics Letters. 51, 913 (1987)) In 1990, Burroughes et al. of Cambridge University found that certain polymer materials showed excellent electroluminescent characteristics and fabricated the first polymer light-emitting diodes, thus extending the development of organic light-emitting diodes to the polymer field. In the past ten years, many efforts have been made to improve the device performance.
At present, OLEDs have not been commercialized in respect to mass production technology. The main difficulties in the mass production of OLEDs are: (1) the problem of OLEDs' lifetime; (2) the problems of production technology and quality control; (3) the problems of related technology (especially drive technology). The lifetime has been one of the hardest difficulties selected from all these problems. That is to say, the lifetime problem of OLEDs has thus far prevented the realization of their full potential to form the next-generation emissive displays and to achieve the commercial production. The lifetime of OLEDs is closely associated with the encapsulation effects because of the oxygen and moisture sensitivity of organic layers and cathodes in the devices. Many researchers have also demonstrated that moisture and oxygen are the main reasons to cause OLEDs' degradation. We may calculate how the oxidation of the low work function (Mg or Ca) cathode limits the operation lifetime of OLED devices by making a simple assumption. For example, assuming a Mg cathode has a thickness of 50 nm, a density of 1.74 g/cm2 and a molar mass of 24 g, the OLED contains 3.6×10−7 mol/cm2 of metallic Mg. Such a cathode can be completely oxidized by about 6.4×10−6 g of water. To achieve a device lifetime of a year, therefore, moisture permeability of an encapsulation layer has to be about 1.5×10−4 g/m2/day or less. Severe device degradation will likely be observed after 10% of the cathode is oxidized (indeed, if the damage occurs at the interface of organic layer and metal layer the metal layer loss of only 5 Å is possible to result in device failure). It is generally considered that the calculation ignores potential degradation of the organic semiconductor itself, which may be catalyzed by water and oxygen. It is clear that long life of OLEDs requires an encapsulation layer which transmits <10−5 g/m2/day of water. (Burrows P E, Graff G L, Gross M E, et al. Displays 22, 65 2001)
Nowadays, OLEDs have reached primary level of industrialization with both organic small molecule emission materials and polymer materials. One area of intensive research and development is flexible displays, which could be made by OLEDs as all-solid display devices with either small molecule or polymer. The displays of this kind, combined flexible transistor technology, could be applied to the products such as e-papers, wallpaper televisions, wearable monitors, etc. The main difference between flexible OLEDs and general ones is substrates which greatly affect the efficiency and operation lifetime of OLEDs. Compared with glass substrates, plastic substrates have the following drawbacks.
(1) The surface roughness of the plastic substrates is worse generally than that of glass substrates. OLEDs will be damaged by the spikes on the plastic surface which can yield pin holes in the functional thin layers.
(2) The plastic substrates have the shortcomings of comparatively high permeation rate of moisture and oxygen which cause the rapid degradation of OLEDs. The requirement in preventing devices from permeating moisture and oxygen is much more exigent than that of the anti-moisture packing film in foodstuff industry.
In order to overcome the shortcomings, much improvement has been made in the aspect of the surface roughness and the barrier properties to moisture and oxygen of the plastic substrates. A new technology called Polymer Multilayer (PML) is considered to be prospective to improve the plastic substrates' performance, which is mentioned in U.S. Pat. No. 4,842,893 (date of patent: Jun. 27, 1989), U.S. Pat. No. 4,954,371 (date of patent: Sep. 4, 1990) and U.S. Pat. No. 5,260,095 (date of patent: Nov. 9, 1993). PML includes a period number of alternating layers of polymer, which can planarize plastic substrates as a flexible buffer layer, and ceramic materials, which serve as a barrier layer, all of which are fabricated in vacuum ambience. The technology involves the flash evaporation of a liquid monomer in room temperature, e.g. acrylic monomer, onto a plastic substrate. Immediately after evaporation, the monomer is cured by irradiation of ultraviolet light, yielding a highly cross-linked and flat polymer film, e.g. polyacrylate. The ceramic barrier layers between polymer films are made up of silicon oxide, silicon nitride, silicon nitrogen oxide, aluminum oxide, aluminum nitride and aluminum nitrogen oxide, etc. which have an extraordinarily low permeation rate of moisture and oxygen and a very high optical transmission in visible spectrum. It has been demonstrated by many investigations that the surface of the plastic substrate is flat enough to fabricate high performance OLEDs, and that the moisture and oxygen permeability of a PML is approximately the same as glass. Because the PML structure is applied on the substrate, the cohesive connection between the plastic substrate and the transparent conductive film (such as Indium Tin Oxide, ITO) is improved, so that the OLEDs' performance is also improved.
The PML structure can also be applied to the encapsulation of OLEDs. The common encapsulation method applied to OLEDs with glass substrates is unsuitable for flexible OLEDs because cathode layers may be damaged by the glass encapsulation sheet for the bending of flexible OLEDs. The PML structure, the period number of alternating polymer and ceramic layers, has been adopted for the encapsulation of OLEDs, which mentioned in U.S. Pat. No. 6,146,225 (date of patent: Nov. 14, 2000). This structure is also inserted by a drier layer made up of active metals. The thin encapsulation layer is next to the cathode of OLEDs and greatly improves the operation lifetime of OLEDs. But the. encapsulation effects of the PML structure alone are not good enough for preventing moisture and oxygen permeation and for improving the mechanical performance such as the flexibility, the ability of preventing scratch and damage of the external force, as desired for commercially viable OLED devices. The thin encapsulation layer and the organic functional layers of OLEDs therefore can be easily degraded.